Cell potential dependence on concentration

In the discussion of the Daniell cell we indicated that this cell produces 1.10 volts. This voltage is really the difference in potential between the two half-cells. There are half-cell potentials associated with all half-cells A list of all possible combinations of half-cells would be tremendously long. Therefore, a way of combining desired half-cells has been developed. The cell potential (really the half-cell potentials) depends on concentration and temperature, but initially we ll simply look at the half-cell potentials at the standard temperature of 298 K (25°C) and all components in their standard states (1 M concentration of all solutions, 1 atmosphere pressure for any gases, and pure solid electrodes). All the half-cell potentials are tabulated as the reduction potentials, that is, the potentials associated with the reduction reaction. The hydrogen half-reaction has been defined as the standard and has been given a value of exactly 0.00 V. All the other half-reactions have been measured relative to it, some positive and some negative. The table of standard reduction potentials provided on the AP exam is shown in Table 16.1 and in the back of this book. 

Because cell potentials depend on concentration, we can construct galvanic cells in which both compartments contain the same components but at different concentrations. For example, in the cell in Fig. 11.11 both compartments contain aqueous AgNC>3, but with different molarities. Let s consider the potential of this cell and the direction of electron flow. The half-reaction relevant to both compartments of this cell is... 

Contemporary pH meters use single probes that contain two reference electrodes, shown diagrammatically in Figure 19-17Z). One electrode contains a buffer solution of known pH. A glass membrane separates this buffer solution from the solution whose pH is to be measured, so this electrode is called a glass electrode. Because hydronium ions participate in the cell reaction of the glass electrode, the overall cell potential depends on the hydronium ion concentration in the solution whose pH is being measured. 

The numerical values of cell potentials and half-cell potentials depend on various conditions, so tables of standard reduction potentials are true when ions and molecules are in their standard states. These standard states are the same as for tables of standard enthalpy changes. Aqueous molecules and ions have a standard concentration of 1 mol/L. Gases have a standard pressure of 101.3 kPa or 1 atm. The standard temperature... 

The cell potential E (also called the cell voltage or electromotive force) is an electrical measure of the driving force of the cell reaction. Cell potentials depend on temperature, ion concentrations, and gas pressures. The standard cell potential E° is the cell potential when reactants and products are in their standard states. Cell potentials are related to free-energy changes by the equations AG = —nFE and AG° = —mFE°, where F = 96,500 C/mol e is the faraday, the charge on 1 mol of electrons. 

So far we have only looked at galvanic cells under standard conditions. Nevertheless cell potentials depend on the concentration of the ions that are in the half cells. E.g. the following overall cell reaction ... 

Shows how the cell potential depends on the concentrations of the cell components ... 

The cell potential of any voltaic cell is positive. The magnitude of the cell potential depends on the reactions that occur at the cathode and anode, the concentrations of reactants and products, and the temperature, which we will assume to be 25 C unless otherwise noted. In this section we focus on cells that are operated at 25 C under standard conditions. Recall from Table 19.2 that standard conditions include 1 M concentrations for reactants and products in solution and 1 atm pressure for gaseous reactants and products. The cell potential under standard conditions is called either the standard cell potential or standard emf and is denoted For the Zn-Cu voltaic cell... 

According to this model the overall cell potential depends on four terms. The first term contains the standard potentials and describes the dependence on the identity of the chemical species in the two flames. The second term describes the Nemstian dependence on the concentrations of the different species at the electrode surface. The third term describes the potential dependence on the electrode kinetics, and the final term is the diffusion potential. 

An electrode potential depends on concentrations of the electrode substances, according to the Nemst equation. Because of this relationship, cell emfs can be used to measure ion concentrations. This is the basic principle of a pH meter, a device that measures the hydrogen-ion concentration. 

The measurement and reference electrode half-cell potentials in Equation 4-lj, which originate from an electrochemical reaction between the internal electrode element and fill, are of opposite sign and should ideally be equal so that their sum is zero. However, the half-cell potentials depend on the type of internal electrode, the internal fill concentration, and the electrode temperature. If the internal electrode element and fill are identical for the measurement and reference electrodes, then the change in half-cell potential with temperature will cancel out unless a temperature gradient exists. The measurement electrode normally has a silver-silver chloride internal electrode in a chloride buffer. The most common type of reference electrode also has a silver-silver chloride internal electrode and a... 

Consider again the electrochemical reaction described by Equation 17.17. If Mj and M2 electrodes are pure metals, the cell potential depends on the absolute temperature T and the molar ion concentrations and [M2 ] according to the Nernst equation,... 

Charge Transport. Side reactions can occur if the current distribution (electrode potential) along an electrode is not uniform. The side reactions can take the form of unwanted by-product formation or localized corrosion of the electrode. The problem of current distribution is addressed by the analysis of charge transport ia cell design. The path of current flow ia a cell is dependent on cell geometry, activation overpotential, concentration overpotential, and conductivity of the electrolyte and electrodes. Three types of current distribution can be described (48) when these factors are analyzed, a nontrivial exercise even for simple geometries (11). 

As a consequence, the equilibrium potential of the single half-cell also depends on the concentrations of the compounds involved. The Nernst equation [Eq. (24)], which is one of the most important electrochemical relations, explains this context... 

As seen in Equations (14.2) and (14.4), the potential of cells and half-cells is dependent on the concentrations of the dissolved species involved. Clearly, the measurement of a potential can lead to the determination of the concentration of an analyte. This, therefore, is the basis for all quantitative poten-tiometric techniques and measurements to be discussed in this chapter. 

It is possible to select a cell that contains a weak acid in solution whose potential depends on the ion concentrations in the solution and hence on the dissociation constant of the acid. As an example, we will consider acetic acid in a cell that contains a hydrogen electrode and a silver-silver chloride electrode ... 

The overall rate of an electrochemical reaction is measured by the current flow through the cell. In order to make valid comparisons between different electrode systems, this current is expressed as cunent density,/, the current per unit area of electrode surface. Tire current density that can be achieved in an electrochemical cell is dependent on many factors. The rate constant of the initial electron transfer step depends on the working electrode potential, Tlie concentration of the substrate maintained at the electrode surface depends on the diffusion coefficient, which is temperature dependent, and the thickness of the diffusion layer, which depends on the stirring rate. Under experimental conditions, current density is dependent on substrate concentration, stirring rate, temperature and electrode potential. 

The numerical value of an electrode potential depends on the nature of the particular chemicals, the temperature, and on the concentrations of the various members of the couple. For the purposes of reference, half-cell potentials are taken at the standard states of all chemicals. Standard state is defined as 1 atm pressure of each gas (the difference between 1 bar and 1 atm is insignificant for the purposes of this chapter), the pure substance of each liquid or solid, and 1 molar concentrations for every nongaseous solute appearing in the balanced half-cell reaction. Reference potentials determined with these parameters are called standard electrode potentials and, since they are represented as reduction reactions (Table 19-1), they are more often than not referred to as standard reduction potentials (E°). E° is also used to represent the standard potential, calculated from the standard reduction potentials, for the whole cell. Some values in Table 19-1 may not be in complete agreement with some sources, but are used for the calculations in this book. 

The potential of a half-cell or cell is dependent on the concentrations of the reagents involved. The... 

Because the cell potential is sensitive to the concentrations of the reactants and products involved in the cell reaction, measured potentials can be used, to determine the concentration of an ion. A pH meter is a familiar example of an instrument that measures concentration from an observed potential. The pH meter has three main components a standard electrode of known potential, a special glass electrode that changes potential depending on the concentration of H+ ion in the solution into which it is dipped, and a potentiometer that measures the potential between the two electrodes. The potentiometer reading is automatically converted electronically to a direct reading of the pH of the solution being tested. 

Is the following statement true or false Concentration cells work because standard reduction potentials are dependent on concentration. Explain. 

The potentials of both individual electrodes are dependent on the hydroxyl ion activity (or concentration) of the potassium hydroxide solution employed as electrolyte. It is evident, however, that in theory the E.M.F. of the complete cell, which is equal to — E+, should be independent of the concentration of the hydroxide solution. In practice a small variation is observed, viz., 1.35 to 1.33 volts for n to 5 N potassium hydroxide this is attributed to the fact that the oxides involved in the cell reactions are all in a hydrousor hydrated form, with the result that a number of molecules of water are transferred in the reaction. The equations for the potentials of the separate electrodes should then contain different terms for the activity of the water in each case the e.m.f. of the complete cell thus depends on the activity of the water in the electrolyte, and hence on the concentration of the potassium hydroxide. 

As explained before, the open-circuit potential of the battery depends on concentration, temperature, and transport limitations. The real voltage delivered by a battery in a closed circuit is affected by ohmic limitations (ohmic potential), concentration limitations (concentration overpotential), and surface limitations (surface overpotential). The close circuit potential of the cell is given by the open-circuit potential of the cell minus the drop in potential due to ohmic potential, concentration overpotential, and surface overpotential. The ohmic potential is due to the ohmic potential drop in the solution. It is mostly affected by the applied charge/discharge current of the battery. The concentration overpotential is associated with the concentration variations in the solution near the electrodes. It is strongly affected by transport properties such as electrolyte conductivity, transference number, and diffusion coefficients. Finally, the surface overpotential is due to the limited rates of the electrode reactions. 

The numerical value of an electrode potential depends on the nature of the particular chemicals, on the temperature, and on the concentrations of the various members of the couple. For purposes of reference, half-cell potentials are tabulated for standard states of all the chemicals, defined as 1 atm... 

Because electrode potential depends on ion concentrations, it is possible to construct a cell from two half-cells composed of the same material but differing in ion concentrations. Such a cell is called a concentration cell. 

Aldehydes, polycyclic aromatic compounds, benzene, ethene, organic acids some of them are known as potential respiratory, eye and skin irritants, others could mutate cells or cause cancer under specific conditions, dependent on concentration of dose... etc. - and, though there is considerable uncertainty as to the... 

---